Arabinoxylan (AX), also referred to as pentosan, is a major constituent in the cell wall of many plant species. For instance in cereal grains, AX occurs at 5-10% of dry weight of the grains. In general, AX from cereals consists of a backbone of beta-(1-4)-linked D-xylopyranosyl residues (xylose), some of which are mono- or disubstituted with alpha-L-arabinofuranosyl residues (arabinose) (lzydorczyk and Biliaderis, 1995). The ratio of arabinose to xylose (average degree of arabinose substitution) in cereal AX ranges from 0.10 to over 1.0, depending on tissue and plant species. In addition, more minor, substituents can be attached to the xylose residues such as acetyl, alfa-glucuronyl, alfa-4-O-methylglucuronyl, galacturonyl, xylosyl, rhamnosyl, galactosyl, or glucosyl side chains, or short oligosaccharide side chains (lzydorczyk and Biliaderis, 1995; Andersson and Aman, 2001). Hydroxycinnamic acids, mainly ferulic acid, and to a lesser extent dehydrodiferulic acid, p-coumaric acid, or sinapic acid, are present as substituents as well, and they are generally linked to the C—(O)-5 position of terminal arabinose units (lzydorczyk and Biliaderis, 1995; Andersson and Aman, 2001). From a dietary point of view, arabinoxylan has raised interest as some types of oligosaccharides derived by enzymatic or chemical cleavage of arabinoxylan have been shown to exert prebiotic properties (WO2006/002495; Courtin et al. 2008).
Prebiotics are compounds, usually oligosaccharides, that can not be digested by enzymes of the upper gastro-intestinal tract but are fermented selectively by some types of intestinal bacteria in the large intestine (Gibson and Roberfroid, 1995; Roberfroid, 1988; Van Loo, 2004). Ingestion of prebiotics causes a shift in the composition of the intestinal bacterial population, typically characterised by a relative increase in Lactobacillus and Bifidobacterium species. This shift in the intestinal microbiota is associated with improved overall health, reduced gut infections, better absorption of minerals, and suppression of colon cancer initiation (Van Loo, 2004; Macfarlane et al. 2006).
Fermentation of prebiotics by colonic bacteria gives rise to production of short chain fatty acids (SCFA) such as acetate, propionate, butyrate and lactate, which act as electron sinks of respiration in the anaerobic environment of the gut. The presence of SCFA in the intestines contributes to a lower pH, a better bio-availability of calcium and magnesium, and inhibition of potentially harmful bacteria (Teitelbaum and Walker, 2002; Wong et al. 2006). Among the SCFA, butyrate appears to be of greatest interest as butyrate is a preferred energy source for colonocytes (Roediger, 1982), stimulates colon epithelial cells, thereby increasing the absorptive capacity of the epithelium (Topping and Clifton, 2001), and inhibits the growth of colonic carcinoma cells, both in vitro and in vivo (Scheppach et al 1995). The cancer-suppressing properties of dietary fibres appear to correlate with their ability to generate butyrate upon colonic fermentation (Perrin et al. 2001).
The selective stimulation by prebiotics of certain colonic bacteria, such as Lactobacilli and Bifidobacteria, which typically use saccharolytic pathways to fuel their energy needs, is in some cases paralleled by suppression of protein fermentation in the colon (van Nuenen et al. 2003; De Preter et al. 2004; Geboes et al. 2005). Reduced protein fermentation in the colon is a desired outcome, as the amino acid degradation pathways in bacteria result in the production of potentially toxic catabolites such as ammonia, amines, phenols, indoles, and thiols, some of which have been implicated in bowel cancer (Bone et al 1976; Johnson, 1977; Visek 1978) and in exacerbation of diseases such as ulcerative colitis (Ramakrishna et al 1991).
Preparations of xylo-oligosaccharides (XOS, oligosaccharides consisting of β-1,4-linked D-xylopyranosyl units) with predominance of oligosaccharides with a degree of polymerisation (DP) of 2-3 (xylobiose and xylotriose), have been shown to cause a significant increase in the level of Bifidobacteria and SCFA in the faeces and caecum of rats (EP 0265970B1; Campbell et al., 1997; Hsu et al., 2004), and the colon of humans (Okazaki et al., 1990). Such xylobiose-rich XOS preparations also suppress early symptoms of chemical-induced colon carcinogenesis in rats (Hsu et al., 2004) and enhance the absorption of calcium in the colon (Toyoda et al., 1993). A preparation consisting predominantly of arabino-xylo-oligosaccharides (AXOS) with a DP of 3-5 (arabinosylxylobiose, arabinosylxylotriose, arabinosylxylotetraose, and diarabinosylxylotetraose) has also been shown to increase the levels of Bifidobacteria in the intestines of rats and mice (Yamada et al., 1993). Experiments described in WO2006/002495 have provided evidence that AXOS with an intermediate average DP (avDP) ranging from 5 to 50 have better prebiotic properties than AXOS with higher avDP, and are less sweet than AXOS preparations with a lower avDP. Addition of such AXOS preparations to the diet causes a significant increase in the number of Bifidobacteria present in the caecum of chickens, caecum of rats, and faeces of humans (WO2006/002495).
For optimal health promoting effects it is furthermore desirable that hydroxycinnamic acids such as ferulic acids are bound to the arabinose side chains of AXOS compounds. Feruloylated AXOS have antioxidant properties due to the presence of ester-linked ferulic acid moieties (Ohta et al 1997; Katapodis et al 2003; Yuan et al 2005), and antioxidants are well known to exert health-related beneficial effects (Ratnam et al 2006). The ester bond that links hydroxycinnamic acids to arabinose side chains of arabinoxylans are cleaved in alkaline media (Rosazza et al 1995; Lozovaya et al 1999). In this respect, the arabinoxylanoligosaccharide-containing preparation described by Yamada et al (1993) is suboptimal as the alkaline treatment used for extraction of arabinoxylans leads to removal of a large fraction of the ferulic acid side chains.
A further important aspect of AXOS-containing preparations intended for food, food ingredient, or nutritional supplement applications is that it has a pleasant and more preferably a neutral taste, since otherwise it may adversely affect the enjoyment and sensory perception of the intake of a product containing that preparation. Moreover, the presence of coloured impurities in (arabino)xylan-oligosaccharide-containing preparations should be avoided such that the preparation does not or only minimally influences the colour of the food or nutritional supplement product in which it is formulated, and no significant colour and taste change should occur upon heating of the nutritional supplement product in which it is formulated.
It is an object of the present invention to provide (arabino)xylan-oligosaccharide preparations comprising a mixture of xylo-oligosaccharides and arabinoxylo-oligosaccharides wherein at least part of said arabinoxylo-oligosaccharides are substituted with hydroxycinnamic acids. The average degree of polymerisation and the overall average degree of arabinose substitution of said xylo- and arabinoxylo-oligosaccharides being selected in order to obtain optimal gastrointestinal health effects upon ingestion of appropriate quantities of said oligosaccharides. At the same time the said food ingredient has a neutral taste and a white or slightly off white colour, which are stable during thermal processing of the ingredient. This thermal stability of the organoleptic and colour properties of the ingredient is particularly useful in the production of thermally processed foods enriched with such mixture of xylo-oligosaccharides and arabinoxylo-oligosaccharides.